Highly Pathogenic avian Influenza (HPAI) H5 strains are currently causing major morbidity and mortality in poultry populations across Asia, Europe, and Africa and have caused 385 confirmed human infections, with a fatality rate of 63.11% (1, 2)1. Preventive and therapeutic measures against circulating H5N1 strains have received a lot of interest and effort globally to prevent another pandemic outbreak. Influenza A virus poses a challenge as it rapidly alters its appearance to the immune system by antigenic drift (mutating) and antigenic shift (exchanging its components) (3). The current combat strategies against influenza include vaccination and anti-viral drug treatment, with vaccination being the preferred option. The annual influenza vaccine aims to stimulate the generation of anti-hemagglutinin neutralizing antibodies, which confer protection against homologous strains. The current vaccines have met with varying degrees of success (4). The fact that these strategies target the highly variable HA determinant and predicting the major HA types that pose the next epidemic threat, can impose limitations to the current viral strategy. In the absence of an effective vaccine, therapy is the mainstay of control of influenza infection. Therefore, therapeutic measures against influenza will play a major role in case a pandemic arises due to H5 strains. Currently licensed anti-viral drugs include the M2 ion channel inhibitors (Rimantidine and Amantidine) and the neuraminidase inhibitors (Oseltamivir and Zanamivir). The H5N1 viruses are known to be resistant to the M2 ion channel inhibitors (5, 6). Newer strains of H5N1 viruses are being isolated which are even resistant to the neuraminidase inhibitors (Oseltamivir and Zanamivir) (3, 7). The neuraminidase inhibitors also may need high doses and prolonged treatment (3, 8). Hence, alternative strategies for treatment of influenza are warranted. Recently, passive immunotherapy using monoclonal antibodies are being viewed upon as a viable treatment option (9). 1A bibliography is provided at the end of the disclosure.
Testing during an outbreak of an acute respiratory disease can determine if influenza is the cause. During influenza season, testing of selected patients presenting with respiratory illnesses compatible with influenza can help establish whether influenza is present in a specific patient population and help health-care providers determine how to use their clinical judgment for diagnosing and treating respiratory illness. A rapid influenza test helps in the determination of whether to use an antiviral medication. Some tests, such as a viral culture, reverse-transcriptase polymerase chain reaction (RT-PCR) and serological testing are the routine methods, but results may not be available in a timely manner to assist clinicians. At present, most of the rapid diagnostic tests currently in use are monoclonal antibody-based immunoassays. Immunofluorescence (fluorescent antibody staining) is the alternative to rapid influenza diagnostic tests which can be used in many hospital laboratories and generally can yield test results in 2-4 hours. Above all, specific monoclonal antibody generation is fundamental to most currently used rapid, sensitive and cost-effective diagnostic methods.
Hemagglutinin (HA) is the most variable gene of the influenza virus and also the most promising target for generating antibodies. It is synthesized as a precursor polypeptide HA0, which is post-translationally cleaved to two polypeptides HA1 and HA2 linked by a disulphide bond. Monoclonal antibodies against the HA1 are known to neutralize the infectivity of the virus and hence provide good protection against infection (10). However, they are less efficient against heterologous or mutant strains which are continuously arising due to antigenic shift. Also, there is the risk of escape mutants being generated which could cause annual epidemics and occasional pandemics.
The HA2 N-terminal fusion peptide is the most highly conserved region in HA among all influenza A subtypes (11). This HA2 polypeptide is responsible for the fusion of the virus and the host endosomal membrane during the entry of the virus into the cell (12). Part of the HA2 N-terminal fusion peptide is exposed as a surface loop in the precursor molecule (13, 14). As most HA subtypes are cleaved by extracellular enzymes, this surface loop is accessible to antibody on HA0 expressed in the plasma membrane of infected host cells (15).
It would be very useful to have available monoclonal antibodies which broadly recognize and can treat influenza A viruses, especially H5 viruses.